btrfs: cache the failed state when locking extents

Currently if we fail to lock a range we'll return the start of the range
that we failed to lock.  We'll then search down to this range and wait
on any extent states in this range.

However we can avoid this search altogether if we simply cache the
extent_state that had the contention.  We can pass this into
wait_extent_bit() and start from that extent_state without doing the
search.  In the most optimistic case we can avoid all searches, more
likely we'll avoid the initial search and have to perform the search
after we wait on the failed state, or worst case we must search both
times which is what currently happens.
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-io-tree.c

@@ -714,7 +714,8 @@ static void wait_on_state(struct extent_io_tree *tree,
  * The range [start, end] is inclusive.
  * The tree lock is taken by this function
  */
-void wait_extent_bit(struct extent_io_tree *tree, u64 start, u64 end, u32 bits)
+void wait_extent_bit(struct extent_io_tree *tree, u64 start, u64 end, u32 bits,
+		     struct extent_state **cached_state)
 {
 	struct extent_state *state;
 
@@ -722,6 +723,16 @@ void wait_extent_bit(struct extent_io_tree *tree, u64 start, u64 end, u32 bits)
 
 	spin_lock(&tree->lock);
 again:
+	/*
+	 * Maintain cached_state, as we may not remove it from the tree if there
+	 * are more bits than the bits we're waiting on set on this state.
+	 */
+	if (cached_state && *cached_state) {
+		state = *cached_state;
+		if (extent_state_in_tree(state) &&
+		    state->start <= start && start < state->end)
+			goto process_node;
+	}
 	while (1) {
 		/*
 		 * This search will find all the extents that end after our
@@ -752,6 +763,12 @@ void wait_extent_bit(struct extent_io_tree *tree, u64 start, u64 end, u32 bits)
 		}
 	}
 out:
+	/* This state is no longer useful, clear it and free it up. */
+	if (cached_state && *cached_state) {
+		state = *cached_state;
+		*cached_state = NULL;
+		free_extent_state(state);
+	}
 	spin_unlock(&tree->lock);
 }
 
@@ -939,13 +956,17 @@ bool btrfs_find_delalloc_range(struct extent_io_tree *tree, u64 *start,
  * sleeping, so the gfp mask is used to indicate what is allowed.
  *
  * If any of the exclusive bits are set, this will fail with -EEXIST if some
- * part of the range already has the desired bits set.  The start of the
- * existing range is returned in failed_start in this case.
+ * part of the range already has the desired bits set.  The extent_state of the
+ * existing range is returned in failed_state in this case, and the start of the
+ * existing range is returned in failed_start.  failed_state is used as an
+ * optimization for wait_extent_bit, failed_start must be used as the source of
+ * truth as failed_state may have changed since we returned.
  *
  * [start, end] is inclusive This takes the tree lock.
  */
 static int __set_extent_bit(struct extent_io_tree *tree, u64 start, u64 end,
 			    u32 bits, u64 *failed_start,
+			    struct extent_state **failed_state,
 			    struct extent_state **cached_state,
 			    struct extent_changeset *changeset, gfp_t mask)
 {
@@ -964,7 +985,7 @@ static int __set_extent_bit(struct extent_io_tree *tree, u64 start, u64 end,
 	if (exclusive_bits)
 		ASSERT(failed_start);
 	else
-		ASSERT(failed_start == NULL);
+		ASSERT(failed_start == NULL && failed_state == NULL);
 again:
 	if (!prealloc && gfpflags_allow_blocking(mask)) {
 		/*
@@ -1012,6 +1033,7 @@ static int __set_extent_bit(struct extent_io_tree *tree, u64 start, u64 end,
 	if (state->start == start && state->end <= end) {
 		if (state->state & exclusive_bits) {
 			*failed_start = state->start;
+			cache_state(state, failed_state);
 			err = -EEXIST;
 			goto out;
 		}
@@ -1047,6 +1069,7 @@ static int __set_extent_bit(struct extent_io_tree *tree, u64 start, u64 end,
 	if (state->start < start) {
 		if (state->state & exclusive_bits) {
 			*failed_start = start;
+			cache_state(state, failed_state);
 			err = -EEXIST;
 			goto out;
 		}
@@ -1125,6 +1148,7 @@ static int __set_extent_bit(struct extent_io_tree *tree, u64 start, u64 end,
 	if (state->start <= end && state->end > end) {
 		if (state->state & exclusive_bits) {
 			*failed_start = start;
+			cache_state(state, failed_state);
 			err = -EEXIST;
 			goto out;
 		}
@@ -1162,8 +1186,8 @@ static int __set_extent_bit(struct extent_io_tree *tree, u64 start, u64 end,
 int set_extent_bit(struct extent_io_tree *tree, u64 start, u64 end,
 		   u32 bits, struct extent_state **cached_state, gfp_t mask)
 {
-	return __set_extent_bit(tree, start, end, bits, NULL, cached_state,
-				NULL, mask);
+	return __set_extent_bit(tree, start, end, bits, NULL, NULL,
+				cached_state, NULL, mask);
 }
 
 /*
@@ -1598,8 +1622,8 @@ int set_record_extent_bits(struct extent_io_tree *tree, u64 start, u64 end,
 	 */
 	ASSERT(!(bits & EXTENT_LOCKED));
 
-	return __set_extent_bit(tree, start, end, bits, NULL, NULL, changeset,
-				GFP_NOFS);
+	return __set_extent_bit(tree, start, end, bits, NULL, NULL, NULL,
+				changeset, GFP_NOFS);
 }
 
 int clear_record_extent_bits(struct extent_io_tree *tree, u64 start, u64 end,
@@ -1622,7 +1646,7 @@ int try_lock_extent(struct extent_io_tree *tree, u64 start, u64 end,
 	u64 failed_start;
 
 	err = __set_extent_bit(tree, start, end, EXTENT_LOCKED, &failed_start,
-			       cached, NULL, GFP_NOFS);
+			       NULL, cached, NULL, GFP_NOFS);
 	if (err == -EEXIST) {
 		if (failed_start > start)
 			clear_extent_bit(tree, start, failed_start - 1,
@@ -1639,20 +1663,22 @@ int try_lock_extent(struct extent_io_tree *tree, u64 start, u64 end,
 int lock_extent(struct extent_io_tree *tree, u64 start, u64 end,
 		struct extent_state **cached_state)
 {
+	struct extent_state *failed_state = NULL;
 	int err;
 	u64 failed_start;
 
 	err = __set_extent_bit(tree, start, end, EXTENT_LOCKED, &failed_start,
-			       cached_state, NULL, GFP_NOFS);
+			       &failed_state, cached_state, NULL, GFP_NOFS);
 	while (err == -EEXIST) {
 		if (failed_start != start)
 			clear_extent_bit(tree, start, failed_start - 1,
 					 EXTENT_LOCKED, cached_state);
 
-		wait_extent_bit(tree, failed_start, end, EXTENT_LOCKED);
+		wait_extent_bit(tree, failed_start, end, EXTENT_LOCKED,
+				&failed_state);
 		err = __set_extent_bit(tree, start, end, EXTENT_LOCKED,
-				       &failed_start, cached_state, NULL,
-				       GFP_NOFS);
+				       &failed_start, &failed_state,
+				       cached_state, NULL, GFP_NOFS);
 	}
 	return err;
 }

fs/btrfs/extent-io-tree.h

@@ -235,6 +235,7 @@ int find_contiguous_extent_bit(struct extent_io_tree *tree, u64 start,
 bool btrfs_find_delalloc_range(struct extent_io_tree *tree, u64 *start,
 			       u64 *end, u64 max_bytes,
 			       struct extent_state **cached_state);
-void wait_extent_bit(struct extent_io_tree *tree, u64 start, u64 end, u32 bits);
+void wait_extent_bit(struct extent_io_tree *tree, u64 start, u64 end, u32 bits,
+		     struct extent_state **cached_state);
 
 #endif /* BTRFS_EXTENT_IO_TREE_H */

fs/btrfs/extent_io.c

@@ -5001,7 +5001,8 @@ static int read_extent_buffer_subpage(struct extent_buffer *eb, int wait,
 	if (ret || wait != WAIT_COMPLETE)
 		return ret;
 
-	wait_extent_bit(io_tree, eb->start, eb->start + eb->len - 1, EXTENT_LOCKED);
+	wait_extent_bit(io_tree, eb->start, eb->start + eb->len - 1,
+			EXTENT_LOCKED, NULL);
 	if (!test_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags))
 		ret = -EIO;
 	return ret;